1. Field of the Invention.
This invention relates generally to a method and device for down mixing compressed digital audio bit stream, and more particularly to a method and device for down mixing the multiple channels of a compressed audio bit stream into a lesser number of audio channels.
2. Description of the Prior Art
Audio compression techniques are used wherever there is an economic benefit to be obtained by reducing the amount of digital information required to represent the audio signals. Examples are the Dolby AC-3, Digital Theatre Systems (DTS), and the MPEG audio layer compression algorithms. The Dolby AC-3 compression format has been selected as the audio format for the high definition television (HDTV) standard in the U.S. It is also widely adopted for use in Digital Versatile Disk (DVD) films.
The AC-3 digital compression algorithm can encode 5.1 channels of source audio from a pulse code modulation (PCM) representation into a serial bit stream at data rates ranging from 32 kbps to 640 kbps. The 0.1 channel refers to a fractional bandwidth channel intended to convey only low frequency (subwoofer) signals. Typically, a compression ratio of 1:10 can be achieved using the AC-3 algorithm. Typical applications are in satellite or terrestrial audio broadcasting, delivery of audio over metallic or optical cables, or storage of audio on, magnetic, optical, semiconductor, or other storage media.
The AC-3 algorithm achieves a high compression ratio by coarsely quantizing a frequency domain representation of the audio signal. The first step in the encoding process is to transform the representation of audio from a sequence of PCM time samples into a sequence of blocks of frequency coefficients. The individual frequency coefficients are represented in floating point representation as a binary exponent and a mantissa. These exponents are encoded according to an adaptive coding process and fed to a bit allocation process. The mantissas are then quantized, the degree of quantization determined by the bit allocation process.
The decoding process is basically the inverse of the encoding process. A decoder must synchronize to the encoded bit stream check for errors, and de-format the various types of data such as the encoded spectral envelope and the quantized mantissas. The decoding process mainly comprises the following four steps: (1) the spectral envelope is decoded to produce the exponents; (2) the exponents are fed into the bit allocation process; (3) the bit allocation routine determines the number of bits used to unpack and de-quantize the mantissas; and (4) the exponents and mantissas of the frequency coefficients are transformed back into the time domain to produce the decoded PCM time samples. A more detailed description of the decoding process is set forth below.